Using in situ polarization microscopy and in situ Raman spectroscopy, a polymorphic phase transition from the triclinic to the monoclinic crystal form of 7-R-methyl-∆5,10-norethindrone has been observed in acetone solutions at room temperature. The metastable triclinic form is nucleated epitaxially on the stable monoclinic form beyond a threshold supersaturation. The metastable phase transforms into the stable phase by a solidsolute-solid mechanism in acetone. The thermodynamics of ordinary and epitaxial nucleation are discussed.

Various morphologies of two polymorphs of a steroid are observed and correlated to calculated morphologies using the connected net theory. In a previous publication on our model compound, we reported polymorphic epitaxial growth as a result of variations in the supersaturation. Two additional and also unusual types of growth behavior are described in this work: the observation of crystals that have a morphology that does not correspond to its observed polymorph, also called pseudomorphism, and the observation of a particular crystal face on which distinctive faceted pits occurred. The particular kinetic growth behavior of this model compound is explained by the fact that the two polymorphs are partly isostructural and by impurity induced growth hampering.

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